For the purposes of the present specification, the following definitions and other points should be noted.
Sludge is a mixture of water and solid materials. Depending on its water content, sludge may be characterized (when the solids content is e.g 3% or less) as “solids swimming in water”, or (when the solids content is e.g 17% or more) as “stiff cake”.
A tonne of raw 5%-solids sludge contains 50 kg of solids and 950 kg of water. Handling/transporting/treating such sludge in its raw, watery, state is wastefully expensive, and desirably the sludge is de-watered prior to processing. Sludge is routinely de-watered by mechanically squeezing, centrifuging, etc, to reduce its water content.
The amount of water that can be removed by routine de-watering depends on the equipment available and other factors; in the case of paunch-manure, or paunch-sludge, it is routinely economical to de-water paunch-sludge to 17% solids. At that, a tonne of raw 5%-solids paunch-sludge, which contains 950 kg of water, after de-watering to 17%-solids, contains only 244 kg of water: the other 706 kg of water has been squeezed out.
Organic sludge is a mixture of water and organic substances. The term “water”, as used herein, should be understood to include solutions, including saturated solutions, of whatever soluble materials might be present. In the sludges with which the present technology is concerned, the organic substances present in the sludge include a substantial proportion of intact biological cells.
Intact biological cells contain a good deal of water. When sludge that contains cells is sheared, the intact biological cells are torn apart and ripped open. When this happens, at least some of the water inside the cell is released.
The distinction is made between free water and captive water, in the sludge. To illustrate this, the example of a blade of grass will be described. The term “solid”, as used herein, means solid in the sense that a blade of grass is a solid article. However, a solid blade of grass contains a good deal of captive-water, and the term “dry-solid” content of the blade of grass is used to indicate the portion of the blade of grass that remains when the grass has been dried, and all the captive-water has been driven off. The mass of a typical biological cell consists of 30% dry-mass bio-solids and 70% captive-water mass.
The sheared bio-solid material can be dried, e.g by prolonged heating, whereby the captive-water is driven off. The dry solid residue that then remains is the so-called dry-mass of that blade of grass.
Thus, in a typical batch of biological sludge, e.g a batch of paunch-manure from an abattoir, the dry-mass of the bio-solids might be e.g five percent of the overall mass of the sludge. The remaining 95% of the overall sludge is water (including both free-water and captive-water).
Thus, a tonne of untreated 5% paunch-manure comprises 50 kg dry-mass of bio-solids, in which is locked 117 kg of captive-water. The 167 kg of grass (comprising 50 kg of dry bio-solids and 117 kg of captive-water) is present in the tonne of 5% paunch-manure sludge, along with 883 kg of free-water.
Some of the free-water (but (almost) none of the captive-water) can be extracted from the paunch by filtering, mechanical squeezing, centrifuging, or a combination thereof. Typically, the paunch-sludge, after the removal of free-water by centrifuging, will be de-watered to e.g 17% solids; or, in other words, the free-water component has been reduced, in the now de-watered paunch-sludge, from 883 kg to 294 kg. Thus, de-watering transforms the one tonne (1000 kg) of 5%-dry-mass paunch-sludge into 411 kg of 17%-dry-mass paunch-sludge—comprising the 50 kg dry-mass of bio-solids with its 117 kg of captive-water, now mixed with only 294 kg of free-water. The other 589 kg of free-water that was present in the tonne of 5%-paunch-sludge has been squeezed out in order to create the 17%-paunch-sludge.
The paunch-sludge, now de-watered to 17%-solids, is ready to be subjected to violent shearing. As a result of shearing, some of the captive-water locked up in the grass cells is released, as the biological cells are torn open. Typically, shearing paunch-sludge can be effective to free up (i.e to release) e.g 40% of the captive-water held in the grass cells. Thus, after shearing, 60% of the captive-water is still retained within the remaining bio-solid material of the grass, even though that material is no longer in the form of intact bio-cells. (That remaining water can be driven off by heating/drying.)
One of the desired results of violent shearing of sludge is to turn the sludge into a homogeneous liquid. As such, the liquefied paunch-sludge is easy to handle and to transport. Liquefied paunch-sludge would be easy to dispose of, e.g by being pumped (injected, sprayed) onto an agricultural field.
As a result of shearing, the bio-solid (solid organic) material of the cell (with its captive-water content) is torn into small fragments. Typically, after violent energetic shearing, these fragments are small enough, and so well-dispersed in the free-water, that the sheared sludge assumes the characteristics of a thick liquid emulsion, like e.g paint.
Thus, sheared sludge can be regarded as a homogeneous liquid. When a viscosity test is performed on unsheared wet sludge, often the viscosity reading is a measurement only of the viscosity of the free-water content of the sludge, rather than of the sludge as a whole substance, and thus the viscosity varies when samples are taken form different locations in the sludge. With violently sheared sludge, on the other hand, the sludge is so homogeneous that viscosity measurements of small samples of the liquid sludge are all consistent with each other—as they are with an actual liquid.
Paunch-sludge, as a substance, is generally regarded as being of negative value; that is to say, disposing of paunch-sludge carries a cost. Liquefying the paunch-sludge offers the possibility that the sheared material can now have value, e.g as a fertilizer, or at least, shearing the material can reduce the net cost of disposal. However, paunch-sludge is very difficult to liquefy, for the reasons discussed below.
Another material that is produced in abattoirs is the material known as DAF float.
In abattoirs and in meat handling and packing plants generally, a good proportion of the waste that has to be dealt with arises from fatty tissue. Most of the fatty material is present as small lumps or pieces of solid material dispersed in water. Meat plants use a good deal of water, e.g for cleaning, and the small lumps (including very small lumps) of fatty material are borne away in the wash-down water.
The fatty material has to be taken out of the water for disposal purposes. One technique is called Dissolved Air Flotation (DAF). Here, water is mixed with compressed air, which dissolves under pressure. When the air-laden water is released into the waste-water, the dissolved air bubbles out of solution, and a myriad of tiny bubbles rises through the waste-water.
The bubbles attract and pick up the lumps of fatty material dispersed through the wastewater, and carry them to the surface. A scum or froth forms at the surface, in which the pieces and lumps of fatty tissue are contained. This scum or froth is termed DAF-float.
If anything, DAF-float has even less commercial value than paunch-sludge. To minimize disposal costs, it is usual to de-water the DAF-float. In a treatment station that de-waters paunch-sludge to 17%-solids, the DAF-float would typically be de-watered to e.g 35%-solids.
35%-DAF-float has the consistency of cold butter. It is handled and transported as a greasy solid. Generally, it is disposed of in a landfill.
Returning now to paunch-sludge, although it is desirable that 17%-paunch-sludge be sheared, and thereby made homogeneous, and of the consistency of paint, shearing the 17%-paunch is not in fact effective to achieve that degree of liquefaction, or at least not in a commercially-economical short period of time.
When de-watered paunch-sludge is placed in a shearing vessel, what happens is that the shearing blades cut a cavity in the solid material, but the rest of the material in the vessel is not drawn into the blades.
Generally, when shearing sludges, the shearing action can be expected to mix the sludge very thoroughly, and lead to such a degree of homogeneousness that it is impossible to detect differences (i.e any differences, including viscosity differences) between samples, no matter where the samples are taken from over the whole body of sheared sludge. However, that does not happen when the sludge being sheared is 17%-paunch-sludge. The sludge simply resides, in the vessel, where it was deposited, and is not mixed and stirred, or even moved, by the shearing blades.
Another problem when shearing paunch-sludge is that the solids in the sludge create a high resistance force on the blades. The resistance force is proportional to blade speed, and so the blades tend to slow down, which is bad for efficiency, and is likely to shorten the life of the drive components, and especially of the shearing blades.
The reasons for these difficulties with 17%-paunch-sludge may be speculated as follows.
The reason may be connected with the shapes and sizes of the pieces of solid material, in relation to the viscosity of the water in which the pieces are dispersed. The liquid itself, being water, is of very low viscosity. On the other hand, the matted strands of partly-digested grass (which are the major component of paunch-sludge) are held together quite tightly. Thus, as the liquid water is swirled about by the blades, the matted strands of biological material remain held together, the force of the moving water being too weak to detach the individual strands from the matted mass of strands.
The strand of straw or grass, even having been bitten off, and having been partly digested, is quite long, being 25 mm long or more. Primarily for that reason, it can take a good deal of force to detach one strand from the matted mass. The forces arising due to swirling of a very low viscosity liquid like water are barely enough to detach the individual strand.
The fact that the strands have the characteristic shape of being long and thin also adds to the force needed to detach the individual strand from the matted mass. A strand that is 25 mm long would be characterized as “long and thin” if its cross-sectional area is less than 5 sq·mm over more than 70% of its length.
If the strands were shorter, or rounder, they would not be, or might not be, snagged so tightly in the matted mass of strands.
When at least 50% of the solid material of the sludge is in the form of strands that are long and thin—for example, are more than 25 mm long and less than 5 sq·mm in area—the problem is likely to arise that the matted strands are so highly resistant to being drawn out of the mat that shearing is not effective to draw them out. In sludge with strands like that, only a small degree of matting can be enough to resist the pull of the swirling water. The strands are held in the mat more forcefully than can be overcome by the viscosity of the water.